1. Field of the Invention
This invention relates to a lens system in which a positive auxiliary lens system is detachably mounted between a photographic lens and an image surface to change a focal length of the photographic lens, thus making it possible to take a photograph at a wide angle of view.
2. Description of the Invention
Conventional systems for changing the focal length of the photographic lens include a system using a zoom lens and a system using an auxiliary lens. Separately from the system using the zoom lens which can easily form the lens into a large type, a known magnification system, in the system using the auxiliary lens, for switching a photographic angle of view into a wide angle or a subwide angle and a standard or tele includes:
(1) A tele-type front converter is provided in front (on an object side) of a main lens in a wide angle system,
(2) A wide angle type front converter is provided in front of a main lens in a tele-system; and
(3) An auxiliary lens having a negative refractive power is mounted between an image surface at rear of a main lens in a wide angle system to increase the resultant focal length.
The aforementioned system (1) has a disadvantage in that the front converter is large. This is particularly conspicuous if the main lens is in wide angle and therefore, aberration is difficult to be well corrected. Where the wide angle is included in a magnification area as in the present invention, this system is extremely difficult to be employed.
The aforementioned system (2) is advantageous, as compared with the system (1), in preventing the front converter from being formed into a large type but has a disadvantage in that since the main lens is in the tele-system, the lens system is long, and if the front coverter is mounted thereon, the lens system is lengthened.
The aforementioned system (3) uses a lens of a wide angle of view as a main lens, and Petzval's sum thereof is naturally designed to make it small. If a negative auxiliary lens is mounted to form a long focal point lens, the negative Petzval's sum increases to make it difficult to obtain a good image surface and in addition, positive distortion aberration also increases.
In addition to the aforementioned systems, the fourth system is considered in which an auxiliary lens having a positive refractive power is mounted at rear of a standard or narrow angle main lens to shorten the resultant focal length. However, this system is hardly known for the reasons below:
a. Since the auxiliary lens having a positive refractive power is added at rear of the main lens, the positive Petzal's sum increases, and the flatness of an image required by the wide angle lens is deteriorated.
b. In the wide angle lens obtained by adding the auxiliary lens having a positive refractive power at rear of the main lens, the positive refractive power is applied to the rear portion of the lens system and therefore, the negative distortion aberration increases. These drawbacks likely occur in the fourth system, rendering design difficult.
For example, in the lens system disclosed in Japanese Patent Application Laid-Open No. 111,825/81 which employs the aforementioned system, the distance from the main lens to the image surface is constant in case where only the main lens is used and in case where the auxiliary lens system is mounted. However, a main point of the auxiliary lens system has to be arranged backwardly due to said restriction, and this deteriorates the magnification efficiency and the full length of the auxiliary lens system is longer than the full length of the main lens, making the system large. As the result, the maximum angle of view is just 55.degree. and the magnification ratio is less than 1.4. Despite this, deterioration of the image-surface characteristic due to the increase in Petzval's sum and an increase in negative distortion aberration are conspicuous.